The present invention relates to a method for heating semiconductor wafers and to an apparatus for carrying out such a method.
Processes for manufacturing semiconductor devices involve steps in which an impurity is diffused into a wafer, or a coating is formed on at least one surface of a wafer. Generally, such a diffusion of an impurity or a formation of a coating is effected by heating a wafer in an atmosphere within a reaction tube. Heating is achieved by a furnace surrounding the tube. More specifically, an elongated reaction tube is horizontally placed in a furnace surrounding the effective length of the reaction tube, and heated to a predetermined temperature. An appropriate gas, such as N.sub.2, O.sub.2 or H.sub.2, is allowed to pass through the reaction tube to establish a desired atmosphere. Arrays of wafers supported by a holder are then introduced into the reaction tube from one end thereof, allowed to stay there for a predetermined period of time, and then withdrawn from the same end of the tube. By such a heating procedure, an even heating cannot be effected because the integrated heat quantity is different not only from wafer to wafer but, also, from portion to portion in each wafer.
Solutions to the above-mentioned problem are proposed in Japanese Patent Publications No. 43-26809 (claiming a convention priority from a U.S. application Ser. No. 529,288, of Feb. 23, 1966), published on Nov. 18, 1968, No. 48-22540, published on July 6, 1973, and No. 49-26457, published on July 9, 1974. According to these prior art solutions, wafers are passed on wafer holders through an elongated reaction tube heated by a furnace surrounding the tube. Such a prior art approach poses, however, the following problems, which are practically very difficult to overcome. First of all, the wafer holders must be provided with certain mechanical means, such as pinions, for their movement through the heated reaction tube. Such mechanical means must be made of a material resistive to a high temperature at which the wafers are to be processed. If the processing temperature is higher than about 900.degree. C., refractory metal materials can not be used for the construction of such mechanical means, because the intended doping is adversely affected due to impurities from such metallic materials. For processing at high temperatures ranging, for example, from 1100.degree. C. to 1250.degree. C., materials such as, quartz, SiC, polycrystalline silicon or SiN.sub.4, are required. Due to significant thermal distortion at high temperatures and poor fabrication precision of such materials, a smooth movement of the wafers through the high temperature zone is not ensured. Secondly, since the wafers and holders are always moving together with the mechanical means, formation of dust is unavoidable, leading to a reduction in the yield (i.e., an increased production of faulty products). Thirdly, if an endless transporter conveyor system is adopted for the purpose of mass production, it is difficult to protect the wafers and holders from dust when they are moving outside the furnace. For the reasons discussed above, the prior art system in which wafers are moved through an elongated reaction tube, has a limited application and is not applicable to cases wherein a shallow diffusion is desired, as is the case with annealing after ion implantation, or to cases wherein a quick processing at high temperatures (e.g., 1100.degree. C. to 1250.degree. C.) is intended.